This invention relates to fluoropolymers.
Fluoropolymers are a commercially useful class of materials. Fluoropolymers include, for example, crosslinked fluoroelastomers, uncrosslinked fluoroelastomer gums, and semi-crystalline plastics. Fluoropolymers exhibit significant tolerance to high temperatures and harsh chemical environments. They are particularly useful as seals, gaskets, and other molded parts in systems that are exposed to elevated temperatures and/or harsh chemicals. Such parts are widely used in the automotive, chemical processing, semiconductor, aerospace, and petroleum industries, among others.
In general, the invention features co-curable blend compositions that include a peroxide-curable fluoroelastomer gum in combination with a fluoroplastic. The fluoroplastic includes (a) at least one bromine atom, iodine atom, or combination thereof, and (b) units derived from at least one fluorinated monomer. The presence of the bromine and/or iodine atoms enables the fluoroplastic to co-cure with the fluoroelastomer gum, resulting in cured articles having improved physical properties such as tensile strength, tear strength, low temperature retraction, and the like. In one embodiment, the fluoroplastics have a melt flow index (MFI) no greater than five grams per ten minutes (measured at 372xc2x0 C. and 5 kg). A high molecular weight (low MFI) fluoroplastic phase of the blend reduces the flow of the plastic phase while under stress at elevated temperatures. In another embodiment, the fluoroplastics have an average particle size in the latex greater than 100 nm. In another embodiment, the invention provides a method of making a fluoropolymer blend comprising providing a fluoroplastic comprising (i) bromine atoms, iodine atoms, and combinations thereof, and (ii) units derived from a fluorinated monomer, said fluoroplastic having a melt flow index no greater than 5 (measured at 372xc2x0 C. and 5 kg) and/or having an average particle size greater than 100 nm, providing a peroxide curable fluoroelastomer gum, and blending the fluoroplastic and the fluoroelastomer gum. Uses for the blends and articles include o-rings, gaskets, tubing, and seals in applications related to, for example, the automotive and aerospace industries.
The details of one or more embodiments of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the description and from the claims.
The fluoroplastics include bromine atoms, iodine atoms, and combinations thereof, and units derived from at least one fluorinated monomer. In addition, the fluoroplastics have a melt flow index no greater than 5 and/or a particle size in the latex greater than 100 nm. The bromine and iodine atoms enable the fluoroplastic to participate in a cure reaction when combined with a fluoroelastomer gum. The fluoroplastic can be formed of slightly modified tetrafluoroethylene polymer (below about 5 mole percent (mol %) comonomer) or of tetrafluoroethylene copolymers with one or more monomers (at least about 5 mol % comonomer) containing at least one ethylene unsaturation in amounts varying from about 0.001 mol % to about 15 mol %, preferably about 0.01-10 mol %. The fluoroplastic melting points range from about 150-325xc2x0 C., more preferably from about 200-325xc2x0 C., and most preferably from about 230-315xc2x0 C. This high melting point enhances high temperature performance properties of the cured blend.
Examples of suitable monomers for the fluoroplastic include olefins, fluoroolefins, and perfluoroolefins (e.g., tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride, and ethylene) and perfluorovinyl ethers of the formula CF2=CF(OCF2CF(CF3))m(O(CF2)n)pORf where m=0-2, n=0-6, p=0-6, and Rf is a C1-C6perfluoroalkyl group. Specific examples include perfluoromethylvinyl ether (PMVE; m=0, n=0, p=0, Rf=CF3), perfluoromethoxypropylvinyl ether (PMPVE; m=0, n=3, p=1, Rf=CF3), perfluoropropylvinyl ether (PPVE-1; m=0, n=0, p=0, Rf=CF2CF2CF3), perfluoropropylvinyl ether-2 (PPVE-2; m=1, n=0, p=0, Rf=CF2CF2CF3), and combinations thereof. Perfluoroalkoxyvinyl ethers may also be included, alone or in combination with other fluorinated monomers. Preferred monomers include perfluoroolefins and perfluorovinyl ethers.
The bromine and iodine atoms may be incorporated in the fluoroplastic in several ways. In one embodiment, the fluorinated monomer(s) is copolymerized with a bromine- or iodine-containing monomer such as bromotrifluoroethylene (BTFE), bromine- or iodine-containing perfluorovinyl ether, 4-bromoperfluorovinylbutene and 4-bromo-3,3,4,4-tetrafluoro-1-butene. In another embodiment, the fluorinated monomer(s) is polymerized in the presence of a bromine- or iodine-containing chain transfer agent. Useful chain transfer agents include diiodomethane, dibromomethane, dibromoperfluoromethane, 1,4-diiodoperfluorobutane, and 1,4-dibromoperfluorobutane.
An effective amount of cure site monomer is used in the fluoroplastic to achieve the desired results. This amount is increased to increase bonding with the fluoroelastomer and this amount is reduced to minimize fluoroplastic modification. The amount of cure site monomer in the fluoroplastic preferably ranges from at least about 0.001 mol %, more preferably at least about 0.01 mol %. The amount of cure site monomer in the fluoroplastic preferably ranges from below about 5 mol %, more preferably below about 2 mol %.
The fluoroplastics preferably are polymerized by aqueous emulsion polymerization in the presence of non-teleogenic fluorine containing surfactants. The polymer particle size is normally in the range of about 100-500 nm (or even between about 100 and 250 nm) after polymerization.
The bromine and/or iodine can be incorporated into the inventive fluoroplastic by incorporating the chain transfer agent(s) and/or cure site monomer(s) either continuously throughout the polymerization, or as a bulk addition in the pre-charge, or more preferably via a core-shell method. One useful core-shell polymerization method involves an emulsion polymerization in which, initially, at least 80 weight percent (wt %) (preferably at least 90 wt %) of a fluorinated monomer-containing composition is polymerized, after which a bromine and/or iodine source is introduced into the reactor, and copolymerizes with the fluorinated polymer. The net result is a fluoroplastic that may have a core-shell structure in which the core predominantly contains units derived from the fluorinated monomer and the shell contains the bromine and/or iodine-containing cure sites.
The fluoroplastic may be combined with a fluoroelastomer gum and a curative composition to form a curable blend. The amount of fluoroplastic in the blend is typically from about 1-50 wt. %, preferably from about 5-50 wt. %, and more preferably from about 10-30 wt. % based upon the total weight of the blend. Fluoroplastics are differentiated from fluoroelastomers by having a melting point which is clearly evident by differential scanning calorimetry.
The fluoroelastomer gum preferably includes bromine and/or iodine atoms. Examples of suitable fluoroelastomer gums are described in U.S. Pat. No. 3,937,690, U.S. Pat. No. 4,035,565 and U.S. Pat. No. 4,243,770.
Useful curatives include peroxides. Examples of useful peroxides include dialkyl peroxides, with di-tertiary butyl peroxides being particularly preferred. Specific examples include 2,5-dimethyl-2,5-di(tertiarybutylperoxy)-hexyne-3 and 2,5-dimethyl-2,5-di(tertiarybutylperoxy)-hexane. Additional examples of useful peroxides include dicumyl peroxide, dibenzoyl peroxide, tertiarybutyl perbenzoate, and di[1,3-dimethyl-3-(tertiarybutylperoxy)-butyl]carbonate.
One or more crosslinking co-agents may be combined with the peroxide. Examples include triallyl cyanurate; triallyl isocyanurate; tri(methallyl)-isocyanurate; tris(diallylamine)-s-triazine, triallyl phosphite; N,N-diallyl acrylamide; hexaallyl phosphoramide; N,N,Nxe2x80x2Nxe2x80x2-tetraallyl terephthalamide; N,N,Nxe2x80x2,Nxe2x80x2-tetraallyl malonamide; trivinyl isocyanurate; 2,4,6-trivinyl methyltrisiloxane; and tri(5-norbornene-2-methylene) cyanurate
The curable blend can also include fillers to improve the physical properties of both the curable and the cured blend. Examples of suitable fillers include reinforcing agents (e.g., thermal grade carbon blacks or non-black pigments), silica, graphite, clay, talc, diatomaceous earth, barium sulfate, titanium oxide and combinations thereof. Other ingredients that may be added to the composition, alone or in combination with one or more fillers, include, for example, metal oxides, plasticizers, lubricants, retarding agents, processing aids, pigments, and combinations thereof.
Thus, the invention provides a composition comprising a fluoroplastic comprising (i) bromine atoms, iodine atoms, and combinations thereof and (ii) units derived from a fluorinated monomer, said fluoroplastic having an MFI no greater than 5 and/or an average particle size greater than 100 nm, a peroxide curable fluoroelastomer gum, a peroxide curative, optionally a coagent, and optionally one or more fillers or additives.
One preferred method of preparing the co-curable blend of this invention is via co-coagulation of the fluoroelastomer and fluoroplastic lattices in the desired ratios. Latex blending is preferred because it produces a substantially homogeneous blend in which the fluoroelastomer gum and fluoroplastic are intimately and uniformly mixed.
The fluoroelastomer gum and fluoroplastic latices are preferably prepared using aqueous-based emulsion polymerization techniques. Suitable polymerization initiators for this purpose include permanganate initiators, with potassium permanganate being particularly preferred, and persulfate initiators, with ammonium and potassium persulfates being particularly preferred. Sulfinates may also be used as part of the initiating system, as described in U.S. Pat. Nos. 5,285,002 and 5,378,782. The fluoroplastic can be produced by incorporating the reactants homogeneously, or as core-shell materials where at least 75% of the composition is polymerized before adding substantial amounts of the bromine/iodine-containing comonomer or chain transfer agent.
The latices may also be coagulated by known methods, including methods involving the addition of salts such as magnesium chloride, aluminum sulfate, and the like, and salt-free methods such as ultrasonic and freeze coagulation.
Before curing, the coagulated, washed and dried curable blend is combined with a curative, preferably by milling the finely divided solids into the gum stock. However, other conventional rubber mixing devices, such as Banbury mixers, can be used as well. To avoid premature curing or xe2x80x9cscorch,xe2x80x9d the temperature during mixing should not rise above about 120xc2x0 C.
The molding and curing process typically involves compression molding the compounded blend in a mold e.g., a cavity or a transfer mold, followed by oven curing. Compression molding of the compounded blend (press cure) is typically conducted at a temperature between about 95xc2x0 C. and about 230xc2x0 C., preferably between about 150xc2x0 C. and about. 205xc2x0 C., for a period of from 5 minutes to about 5 hours, usually from 5 minutes to 30 minutes. A pressure of between about 500 kPa and about 15,000 kPa, preferably between about 4,000 kPa and about 8,000 kPa, is imposed on the compounded mixture in the mold. The molded vulcanizate is then usually post cured (oven cured) at a temperature between about 150xc2x0 C. and about 260xc2x0 C., usually at about 232xc2x0 C., for a period of from about 2 hours to 30 hours or more depending on the cross-sectional thickness of the sample. For thick sections, the temperature during the post cure is usually raised gradually from the lower limit of the range to the desired maximum temperature selected. For thinner cross-sections, e.g., less than 5 mm, the vulcanizate or cured sheet section may be put into the oven at the desired maximum temperature.